Counteracting a plant's defense: Suppression of herbivore-induced plant volatiles and phytohormones by pea aphids

Background/Question/Methods   Plants are able to defend themselves against herbivory through several means, including the production of airborne volatile organic compounds (VOCs). These VOCs benefit plants by attracting natural enemies of their herbivores. In addition to airborne VOC signals that are dispersed into the environment, plants also respond to insect herbivory by ramping up the production of within-plant defense molecules, including the pytohormone Jasmonic Acid (JA). The pea aphid, Acyrthosiphon pisum, is advantaged as an herbivore when feeding on its host plant, Vicia faba, in that it is able to feed without inducing detectable changes in plant VOC emission. Aphids are also able to avoid triggering the precursory phytohormone (JA) that is associated with herbivory. The aim of our study was to examine whether or not aphids simply avoid triggering plant defense signaling, or if they are able to manipulate plant signaling in away to alter effectual plant defenses.

Results/Conclusions   We found that pea aphids have the ability to inhibit the release of VOCs as well as accumulation of the precursory phytohormone JA. Levels of VOCs emission and hormone accumulation are not significantly different between control plants and those fed upon by aphids for up to 5 days. Using a second herbivore, the beet armyworm caterpillar, Spodoptera exigua, we demonstrate that expected caterpillar-induced VOCs and phytohormones are suppressed when co-infested with pea aphids. We see an expected increase in both VOC emission and JA accumulation when plants are subject to herbivory by the beet armyworm caterpillar, however this induction is suppressed when pea aphids colonize plants prior to caterpillar feeding. We have identified a potential mechanism for this suppression and will discuss its source and mode of action.